This paper presents an approach to evaluate the Unmanned Aircraft System (UAS) globally harmonized spectrum. The review of UAS consists about the roadmap and the previous plans of compatibility research study. It leads to an approach, which proposes a model of interference as a method to assess the compatibility of incumbent system towards the new system. ITU has studied the potential use in 5000-5150 MHz. This frequency band is occupied mainly with aircraft-related communications. However, the previous planned spectrum has not yet supported BLoS signal. The problems are lack of satellite system in 5000-5150 MHz and lack of bandwidth in 960-1164 MHz and 5000-5150 MHz. Therefore, we give our attention to another spectrum such as FSS spectrum that is able to support large bandwidth and to provide satellite system. We identify the issues in BLoS radio signal and the parameter of compatibility. It yields a workflow that has an input, the parameters of compatibility and an output. The model we propose has a basis in interference model between two systems.